6.7
Presión osmótica
La ósmosis es un proceso en el que las moléculas de disolvente se mueven hacia una solución a través de una membrana semipermeable. A medida que la so…
La ósmosis es el movimiento del disolvente a través de una membrana semipermeable hacia una solución con una concentración de soluto más alta.
A medida que la solución se diluye y expande debido al disolvente añadido, su presión hidrostática, que es la presión ejercida por un fluido en equilibrio debido a la gravedad, aumenta, deteniendo finalmente la ósmosis.
La presión osmótica, Π, es la presión que debe ejercerse sobre una solución para evitar la entrada de disolventes.
El Π de una solución ideal se calcula usando la ecuación de Van't Hoff, que correlaciona Π con la concentración del soluto.
No obstante, las soluciones de macromoléculas como los polímeros no son ideales debido a los efectos de volumen excluido y a las interacciones polímero-polímero. Como resultado, sus masas molares, M, se calculan usando la ecuación de Van't Hoff expandida que implica el coeficiente viral osmótico, B.
Para simplificar esta ecuación, ambos lados se dividen por la concentración molar del polímero J, que es la relación entre la concentración de masa, masa c, J y M.
Graficarla masa Π/c, J frente ala masa c, J en varias concentraciones de J permite estimar el valor M desde el intercepto y, después, el valor B desde la pendiente.
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Q1: What is osmotic pressure and how does it form?
Osmotic pressure, denoted Π, is the minimum pressure required to prevent solvent from moving across a semipermeable membrane into a solution. It develops as solvent molecules move toward higher solute concentrations, diluting the solution and increasing its hydrostatic pressure until equilibrium is reached and osmosis stops.
Q2: How does the van't Hoff equation calculate osmotic pressure?
The van't Hoff equation correlates osmotic pressure with solute concentration for ideal solutions, where solute-solvent interactions match those among solvent molecules. This equation provides a straightforward method to compute osmotic pressure from known concentrations, making it fundamental for understanding colligative properties.
Q3: Why do polymer solutions require a modified van't Hoff equation?
Polymer solutions are non-ideal due to excluded volume effects, where polymer chains cannot occupy certain spaces due to unfavorable overlapping, causing them to spread out more than ideal chains. Additionally, polymer-solvent interactions differ from solvent-solvent interactions, necessitating an expanded van't Hoff equation with the osmotic virial coefficient B.
Q4: How is polymer molar mass determined from osmotic pressure data?
By plotting osmotic pressure divided by mass concentration (Π/cmass,J) versus mass concentration (cmass,J) at various polymer concentrations, the molar mass M is determined from the y-intercept of the resulting line. The osmotic virial coefficient B is then calculated from the slope.
Q5: What role does hydrostatic pressure play in stopping osmosis?
As solvent enters the solution through the semipermeable membrane, the solution expands and its hydrostatic pressure increases. When hydrostatic pressure equals osmotic pressure, the driving force for solvent movement ceases, halting osmosis and establishing equilibrium across the membrane between solutions.
Q6: What is excluded volume effect in polymer solutions?
Excluded volume effect describes the space a polymer chain cannot occupy due to unfavorable chain overlapping. This causes polymer molecules to be more spread out than ideal polymer chains would be, affecting their osmotic behavior and requiring corrections to the van't Hoff equation for accurate molar mass calculations.
Q7: How does osmosis differ between ideal and nonideal two component liquid solutions?
Ideal solutions follow the van't Hoff equation directly because solute-solvent interactions match solvent-solvent interactions. Nonideal two component liquid solutions like polymer systems deviate due to excluded volume effects and different intermolecular interactions, requiring the expanded van't Hoff equation with the osmotic virial coefficient for accurate pressure calculations.